Ballast circuits for fluorescent lamps are most commonly in the form of a coil/core construction suitable for operation at low frequencies. Recently, electronic ballasts employing much higher frequencies have been designed and employed in various commercial, industrial and residential applications. These recent electronic ballasts offer greatly improved efficiency while providing such capability with structures of reduced size and weight.
Unfortunately, these newer electronic ballasts suffer from disadvantages. Primarily the cost of the newer electronic ballasts has increased considerably over prior known structures. Moreover, this undesired increased cost appears to be directly related to an increase in component count which is, in turn, caused by circuitry which converts a low frequency AC supply voltage to a direct current (DC) voltage and again to a high frequency voltage for application to the lamp. Obviously, such multiple conversion is deleterious to a cost effective structure.
One known technique for operating fluorescent lamps without a ballast arrangement is set forth in U.S. Pat. No. 3,771,013 issued to Roche et al on Nov. 15, 1973 and assigned to the assignee of the present application. Herein, a saturated transistor amplifier is utilized to prevent the peak operating current of the lamp from exceeding a predetermined maximum level. However, a problem was found to exist in that the filter capacitor employed had to have a voltage rating equal to the supply voltage multiplied by the .sqroot.2, even though a voltage of such a magnitude is experienced only during the starting period or at the end of life of the fluorescent lamp. Moreover, the voltage experienced by the filter capacitor is much lower during the operational life of the fluorescent lamp. Thus, a filter capacitor having a maximum voltage rating of a value which exists only during the starting and end of life of the fluorescent lamp appears to be needlessly excessive in size and in cost.